The present invention generally relates to fabrication of semiconductor devices, and more particularly to a fabrication of a semiconductor memory device having a floating gate such as a flash memory or EEPROM, with a step of forming an insulation film that is immune to breakdown even with a repeated injection of electric charges into the floating gate.
Semiconductor memory devices such as flash memories or EEPROMs are characterized by a floating gate for storing information in the form of electric charges. Thereby, the semiconductor memory devices are capable of retaining information for a prolonged time period even when an electric power is cut off. In other words, the semiconductor memory devices having a floating gate operates as a non-volatile semiconductor memory.
With increased storage capacity of semiconductor memory devices associated with the progress in the fabrication technology of integrated circuits, there has emerged a prospect that these nonvolatile semiconductor memory devices, particularly the flash memories, may be used for replacing conventional magnetic memories such as flexible disk drives or hard disk drives. Such non-volatile, solid-state semiconductor memory devices, lacking delicate, moving mechanical parts, have an obvious advantage over the conventional magnetic disk drives in terms of the compact and rigid construction. Thereby, the reliability of the memory devices would improve significantly by using such semiconductor non-volatile memories. In addition, the access time would be reduced significantly.
In the conventional non-volatile semiconductor memory devices such as EEPROM, the storage of information has been achieved by injecting electric charges into the floating gate as a tunneling current through an insulating film that isolates the floating gate. On the other hand, such conventional devices are used for limited purposes and the number of times the data is written is relatively limited. When the non-volatile semiconductor devices are going to be used to replace the magnetic disk drives as in the case of recent flash memories, the number of times the data is written increases significantly. Thereby, the degradation of the insulation film that isolates the floating gate becomes a major problem. It is believed that the major factor that causes such a degradation of the insulation film is the current density of the tunneling current that flows through the insulation film, not the electric field that is applied to the insulation film.
Meanwhile, the state-of-art fabrication technology of semiconductor devices can provide an extremely flat surface on a silicon wafer on which the semiconductor memory device is constructed. Such a fabrication process includes a polishing process for finishing the surface of the silicon wafer with a very high precision. The flat surface of the silicon wafer thus processed is provided with a thin oxide film, and such a thin oxide film is used for the tunneling oxide film of the flash memories. Generally, the tunneling oxide film thus formed has an extremely uniform thickness, and a large electric field is necessary for injecting electric charges therethrough to the floating gate electrode.
FIG. 1 shows a conventional MOS diode that includes an insulation film 42 of silicon oxide formed on a surface of a silicon wafer 41. On the insulation film 42, a polysilicon electrode 43 is provided.
As indicated in FIG. 1, the silicon wafer 41 has a flat, finished surface processed by a polishing process with a high precision in the order of 0.1 nm in terms of surface roughness, and the silicon oxide film 42 is formed on such a flat surface of the silicon wafer 41 by a thermal oxidation process with a uniform thickness. The film 42 is formed to have a thickness for allowing electric charges to flow therethrough from the polysilicon electrode 43 to the substrate 41 or vice versa in the form of tunneling current.
It should be noted that current fabrication process cannot eliminate completely the deposition of dust particles or heavy metal deposits on the surface of the silicon wafer 41, and the tunneling insulation film 42 formed on the wafer 41 generally includes some defects 44 scattered over the surface of the wafer 41. When such defects 44 exist, the thickness of the insulation film 42, typically formed by the thermal oxidation process of the substrate 41, may be reduced locally in correspondence such defects. Further, such a tunneling oxide film 42 may have a reduced resistance in correspondence to the defects when the defect is formed of conductive dust particles such as silicide.
When the tunneling oxide film 42 includes such scattered defects 44, an electric current concentrates to localized, low resistance regions that include the defects 44, and such concentration of the electric current tends to cause a trap of carriers in the insulation film 42. Thereby, the magnitude of the electric field increases further, and such an increase of the electric field in turn invites a concentration of the electric current. Ultimately, the tunneling insulation film 42 is broken down.
Thus, conventional semiconductor non-volatile memory devices, fabricated by a process that includes the step of forming a tunneling insulation film on a polished surface of a silicon wafer, has suffered from the problem of limited lifetime of the tunneling insulation film. When the electric charges are injected to the floating gate electrode repeatedly or for a prolonged time period, the tunneling insulation film tends to show a localized breakdown.